1. Field of the Invention
The present invention relates to an optical recording medium recording and reproducing apparatus that records information to an optical recording medium having a first information recording layer and a second information recording layer and reproduces information from the optical recording medium and a tracking control method for performing a tracking control on the optical recording medium.
2. Description of the Related Art
In an optical recording medium recording and reproducing apparatus that records information to an optical disk and reproduces information from the optical disk, a laser beam is converged on a recording plane of the optical disk through a transparent substrate so that the information can be recorded to or reproduced from a target track by a small beam spot of the laser beam with a high density. To have the beam spot follow the target track accurately, a process of tracking servo is performed in which a tracking error is detected based on a reflected beam from the optical disk, and the objective lens is driven based on the detected tracking error signal so that the position of the beam spot is corrected in real-time.
A recordable optical disk such as a Digital Versatile Disk-Recordable (DVD-R) and a Digital Versatile Disk-Rewritable (DVD-RW) has guiding grooves on the disk. These grooves are used as tracks. Examples of tracking error detection methods include a differential push-pull method that uses a plurality of beams, normally, three beams. Even if only one beam is used, it is possible to detect tracking errors based on a push-pull signal indicating an intensity difference between an inner circumference side of the track and an outer circumference side of the track, by causing a reflected beam from the optical disk to converge on a light detecting unit.
However, if the objective lens shifts away from the center of a beam path due to an eccentricity of the optical disk or the like during the tracking process, an offset will be caused in a subtraction calculation of the push-pull signal. To cope with this problem, a tracking servo technique that uses the differential push-pull method is known by which the push-pull signal is detected by using a plurality of beams at the same time, and a calculation is performed so as to cancel the offset.
In recent years, attention is paid to increases in the capacity of single-sided multi-layer optical disks each of which has a plurality of information recording layers to and from which information can be recorded and reproduced. In an optical recording medium recording and reproducing apparatus used for such single-sided multi-layer optical disks, an optical beam is converged on each information recording layer used for the recording or the reproducing, and a reflected beam from the information recording layer is received, so that it is possible to record or reproduce information from one side of each information recording layer. The tracking servo technique used in this situation is the same as the one used for normal optical disks.
A difference in a single-sided multi-layer optical disk is that all the reflection films except for the reflection film provided on the information recording layer that is positioned farthest from the laser-beam incident side are arranged to be translucent, so that it is possible to record or reproduce information to or from each of the information recording layers from the one side of the disk. Thus, an inter-layer crosstalk is caused in which a reflected beam that has been reflected on an information recording layer (hereinafter, a “non-recording/reproducing information recording layer”) that is different from the information recording layer onto or from which information is to be recorded or reproduced (hereinafter, “recording/reproducing information recording layer”) is irradiated onto a light receiving element in the light detecting unit, in a largely defocused manner.
In other words, the light receiving element in the light detecting unit not only receives a reflected beam from the recording/reproducing information recording layer but also receives the reflected beam from the other non-recording/reproducing information recording layer in a spread and multiplexed manner.
Among the beams used by an optical system included in a commonly-used optical recording medium recording and reproducing apparatus, the ratio of intensity between a main-beam that is used for recording and reproducing information and sub-beams that are used for performing the tracking servo process with the differential push-pull method is approximately 10:1. Thus, in the light receiving areas for the sub-beams, the intensity of the reflected beam of the main-beam on a non-recording/reproducing information recording layer is too high to be negligible, compared to the intensity of the reflected beam of a sub-beam reflected on a recording/reproducing information recording layer, which is supposed to be a target of the detection process. This situation has a negative influence on the signal detection process.
To solve this problem, JP-A 2005-293807 (KOKAI) discloses a conventional technique that reduces the influence of multiplexed reflected beams from non-recording/reproducing information recording layers by making the light receiving areas for the sub-beams smaller in a light detecting unit.
Furthermore, JP-A 2005-353252 (KOKAI) discloses another conventional technique by which a light receiving area that is exclusively for a reflected beam from a non-recording/reproducing information recording layer is additionally provided in each of two positions on the outside of a normal light receiving area that receives a reflected beam from a recording/reproducing information recording layer, so that a sub-beam detection signal is corrected by using a signal in the detected reflected beam from the non-recording/reproducing information recording layer.
However, when the technique disclosed in JP-A 2005-293807 (KOKAI) is used, there is a limit to the extent to which the light receiving area in the light detecting unit can be smaller. Thus, when the light receiving area is not small enough, it is not possible to sufficiently mitigate the influence of the reflected beam from the non-recording/reproducing information recording layer.
Furthermore, when the technique disclosed in JP-A 2005-353252 (KOKAI) is used, the light receiving area that is exclusively for the reflected beam from the non-recording/reproducing information recording layer is additionally provided so that the sub-beam detection signal is corrected by using the signal in the detected reflected beam from the non-recording/reproducing information recording layer. Thus, it is necessary to provide a light receiving element that has a special light receiving pattern so as to provide the light receiving area that is exclusively for the reflected beam from the non-recording/reproducing information recording layer. In addition, it is also necessary to additionally provide a detecting circuit for the signal in the reflected beam from the non-recording/reproducing information recording layer as well as a signal processing circuit for the signal in the reflected beam. Consequently, a problem arises where the structure of the optical recording medium reproducing apparatus becomes complicated.
In particular, in a case where information is not recorded in the entire area of a non-recording/reproducing information recording layer, in other words, information is recorded only in a partial area of a non-recording/reproducing information recording layer, another problem as described below arises.
At the moment when a light beam that is converged on a recording/reproducing information recording layer moves from an area on the recording/reproducing information recording layer that faces an area on a non-recording/reproducing information recording layer in which no information has been recorded to an area on the recording/reproducing information recording layer that faces an area on the non-recording/reproducing information recording layer in which information has already been recorded, a difference is caused in the intensity of the reflected beam from the non-recording/reproducing information recording layer between the inner circumference side of the track and the outer circumference side of the track. The beam is multiplexed onto the sub-beam light receiving area while having the intensity difference. The intensity difference causes an error in the detection process of the push-pull signal in the sub-beams, and also, an error will be caused in a tracking error signal used in the differential push-pull method. As a result, when the tracking servo operation is performed based on mutually the same tracking error signal, a phenomenon called “de-tracking” occurs in which the beam is not able to trace the center of the track of the optical disk. This leads to a problem where it is not possible to record and reproduce the information accurately.